Ammonia is an important supply source of nitrogen and an indispensable nutrient used for fermentation.
An existing technique for measuring ammonia concentration involves a technique of using an ion electrode.
For example, in the method described in WO2006/038695, the ammonia concentration in a liquid fermentation medium is controlled to be below a certain concentration, and thereby fermentation bacteria are cultured at a higher pH value. The counter ions to be added to the medium for the production of a basic substance by fermentation are thereby reduced, and the manufacturing process is significantly simplified as a result.
However, the above-mentioned conventional technique has a problem that it is difficult to directly control the ammonia concentration in the liquid fermentation medium, that is, directly measure or control the ammonia concentration in a culture medium contained in a culture tank during the culture, by that technique.
Namely, it has a problem that, as shown in FIG. 9 (flowchart showing an example of the conventional process of ammonia concentration control), in order to know the total ammonia concentration (total concentration of NH3 and NH4+) in the culture medium, the conventional process requires a series of operations of sampling the culture medium from the inside of the culture tank (Step SA-1), mixing the sampled culture medium with a strongly alkaline reaction mixture (for example, NaOH) (Step SA-2), and continuously measuring the existing ammonia converted into non-ionized ammonia (NH3) out of the culture tank (Step SA-3). That is, it is difficult to sterilely perform the series of operations for the measurement with the sampled culture medium out of the culture tank. Moreover, since the culture medium used for the measurement contains the strongly alkaline reaction mixture (for example, NaOH), it cannot be recycled into the culture tank and must be discarded. Therefore, a higher measurement frequency invites larger waste of the culture medium, and the number of times of actual measurement is limited (Step SA-4).
Furthermore, in the culture tank, ammonia is continuously consumed, but the ammonia consumption rate is not constant. Therefore, it also has a problem that, as shown in FIG. 9, even if the ammonia concentration is measured by the above-mentioned sampling operation with a certain interval (Step SA-5), the short period trend thereof is not known, and therefore the ammonia concentration in the culture medium cannot be controlled to be constant (Step SA-6).
Furthermore, even if the non-ionized ammonia (NH3) partly existing in the culture medium is measured with an ion electrode in view of the aforementioned problems of the conventional technique, it suffers from a problem that it is not easy to proof the measured values for correcting errors occurring during the culture. This is because, in order to know the present correct ammonia concentration, it is necessary to measure the total ammonia concentration for a sampled culture medium mixed with a strongly alkaline reaction mixture so that the ammonia in the culture medium is converted into non-ionized ammonia (NH3).
Namely, that is because, in usual fermentation, the pH value of the culture medium is within a weakly acidic to weakly alkaline range (about pH 5 to 9), thus a part or substantially all of the ammonia existing in the culture medium exists as ionized, ammonium ion (NH4+), and therefore it is difficult to obtain the total ammonia concentration (total concentration of NH3 and NH4+) in the culture medium only by measuring the concentration of non-ionized ammonia (NH3) with the above-mentioned ion electrode. Therefore, the total ammonia concentration cannot be correctly controlled.